With the development of power system technologies, the cost of line-commutated converter based high voltage direct current transmission system (LCC-HVDC) decreases gradually, while the reliability improves and the power loss reduces continually. Currently, line-commutated converter based high voltage direct current transmission system has been widely used in large capacity long distance power transmission, island transmission and asynchronous network back-to-back interconnection. However, the LCC-HVDC has the following three shortcomings:
(1) Commutation failure occurs easily at the inverter. The main device of LCC-HVDC is the half-controlled thyristor, and its commutation depends on AC systems, hence fault or voltage distortion of AC system may lead to commutation failure. This will cause huge impact on the AC system and severely affects system's safe and smooth operation.
(2) Strong dependency on AC system. The LCC-HVDC cannot supply power to weak AC system or passive system.
(3) Large area occupied by converter station. The set point of LCC-HVDC is hard to choose and has big problems.
In summary, the above-mentioned shortcomings have restricted the application of LCC-HVDC.
The modular multilevel converter based high voltage direct current transmission system (MMC-HVDC) is a new type of flexible transmission system, and its basic device is the half-bridge submodule. Compared to the LCC-HVDC, the MMC-HVDC has many advantages, such as being able to control active power and reactive power independently, low switch frequency, low power loss, low distortion of output voltage, low cost of filters, low occupied area and, most importantly, inexistence of commutation failure. As a result, the MMC-HVDC has strong competitiveness in renewable energy integration, large-capacity power transmission and distribution for cities as well as supplying power to passive AC system.
However, the MMC-HVDC has the following shortcomings:
1) The MMC-HVDC cannot effectively deal with DC fault; hence the reliability is comparatively weak. When DC fault occurs, the freewheel diode, which is antiparallel to the full-controlled power electronic device, becomes the path between the energy feed point and the fault point; this will lead to transient overcurrent and must be cut off by tripping the AC mechanical switch. But 2-3 cycles time is the best for the AC mechanical switch to cut off the transient overcurrent; and during this period the current increases greatly, which requires larger rated parameters of equipment and means more construction cost. For this reason, the MMC-HVDC usually abandons overhead line and adopts cable as the transmission path, which is of lower fault rate but larger cost.
2) The design and installation of grounding branch is difficult. There are two grounding modes usually used, namely the reactor mode and the resistor mode. The former adopts star-connected reactors at the AC side, but the value of the reactor is hard to choose and the reactor will influence the operation range of converter reactive power. The latter adopts clamp resistor at the DC side; however, small-resistance will lead to great power loss, while large-resistance will result in poor grounding effect.
3) Compared to LCC-HVDC, the unit investment cost of MMC-HVDC is significantly higher; hence MMC-HVDC is less economical when applied to long-distance large-capacity power transmission.
The above-mentioned shortcomings have restricted the application of MMC-HVDC in engineering practice.